Under conditions of low humidity, low nighttime temperatures, and little or no cloud cover, heat from exposed earth, plants, and the like can quickly radiate skyward. Conversely, when such elements are covered by way of man-made housings or meteorological coverings, e.g., water vapor in the form of high humidity or cloud cover, the heat from the earth and plants is not able to radiate skyward, but rather is dissipated into the ambient air and trapped. The ambient air temperature thus remains relatively high and in turn maintains relatively high surface temperatures in the earth and plants as well.
Analyzing this phenomenon in deeper detail, the microclimate of a typical plant leaf needs to be considered. Such plant leaves have a given temperature based on a number of parameters, including ambient air temperature, earth temperature, plant cycle activity, wind speed, etc. As with any object, if the temperature of the plant leaf is greater than the temperature of the ambient air surrounding the leaf, heat will be radiated and dissipated from the leaf to the ambient air. The leaf will become colder than the air ambient temperature due to a lack of heat exchange from convection. Only the air in direct contact with the leaf is near the leaf temperature.
If the above-referenced conditions of low humidity, ambient air temperature and cloud cover are present, the water vapor will radiate skyward and the heat from the plant will dissipate at a sufficiently high rate that the surface temperature of the leaf reduces in temperature faster than the ambient air temperature. If the surface temperature of the leaf drops below the freezing point of water, microscopic water droplets on the leaf will crystalize into ice, resulting in the well-known condition of frost.
Frost is a damaging condition which agriculturalists fervently attempt to avoid. The crystalline ice structure of frost creates a barrier over the plant leaves, preventing the intake of carbon dioxide, and the release of oxygen which are essential steps for sustaining plant life. If the frost occurs before harvest, crop yields can be diminished, if not completely lost.
Various mechanisms are therefore employed for preventing frost formation. For example, with small-scale gardens, homeowners often cover their plants with plastic or paper shrouds to capture radiant heat if the overnight weather forecast is for relatively low temperatures. Larger facilities such as nurseries, often maintain operations within large glass greenhouses, at least in part, for the same reason. Still larger operations such as farms and vineyards often employ fans at ground level to circulate the ambient air in immediate contact with the ground. This air is slightly warmer, due to the warmth of the earth, and can be used to elevate the temperature of the plants.
Given the economic and productivity impact of frost damage on crops, as well as the cost associated with the aforementioned frost prevention mechanisms, improved apparatus and methods for predicting the onset of frost could be advantageously employed in improving crop yield while reducing cost at the same time.
Thus, there is a need for a method and apparatus for predicting with greater reliability the likelihood of an impending frost, and for taking measures to guard against frost damage.